1. Field of the Invention
The invention relates to a method for manufacturing a spark plug and to an apparatus for manufacturing a spark plug.
2. Description of Related Art
Recently, in order to enhance spark ablation resistance of a spark plug used for providing ignition in an internal combustion engine, a certain type of spark plug has employed a noble-metal spark portion formed by welding a noble metal chip predominantly containing Pt, Ir, or the like, to a distal end of a center electrode formed from an Ni- or Fe-based heat-resistant alloy. For example, to join a noble-metal chip to the distal end face of a center electrode so as to form a spark discharge gap in cooperation with an opposed ground electrode, a full-circled laser-beam welding method has been proposed (as disclosed in, for example, Japanese Laid-Open Patent Applications Nos. H06-45050 and H10-112374). According to the method, a disk-like noble metal chip is attached to the distal end face (a chip joint face) of the center electrode, and the boundary between the chip and the electrode is irradiated with a laser beam while the center electrode is being rotated, thereby forming a full-circled laser-beam weld metal portion.
The above-mentioned method for manufacturing a spark plug employs a pulsed laser beam emitted from a YAG laser or the like for forming a laser-beam weld metal portion along the circumference of the noble metal chip in such a manner as to intrude into the noble metal chip and into a chip joint face formation portion of the center electrode. However, since a melting point difference of about hundreds of ° C. to 1,000° C. exists between the chip material (a noble metal such as Pt or Ir or a noble metal alloy predominantly containing the same) and the electrode material (an Ni- or Fe-based alloy), a slight variation in welding conditions leads to insufficient fusion of a noble metal or noble metal alloy or excessive fusion of an electrode alloy, resulting in a failure to form a sound weld metal portion. Also, when spark plugs of different spark-portion specifications such as different materials for a noble metal chip or center electrode are to be manufactured in the same production line, the difference in spark-portion specifications is apt to greatly influence the quality of weld metal portions.
For example, when the depth of the weld metal portion becomes insufficient as a result of a variation in welding conditions, the weld strength between the center electrode and the noble metal chip is impaired, and thus the separation resistance of a formed spark portion is impaired. Also, when the width of the weld metal portion as measured along the thickness direction of the chip is increased excessively or when the position of the weld metal portion is biased toward the discharge face, the shortest distance as measured along the axial direction of the center electrode between the chip discharge face and the end edge of the weld metal portion, i.e., the spark portion thickness, becomes insufficient. As a result, even slight ablation of the spark portion involves exposure of the weld metal portion at the discharge face, thereby directly shortening service life.